Light-emitting diodes (LED) offer many advantages over conventional lighting apparatus, such as long lifetime, high efficiency, and non-toxic materials. With the development of electronic technology, light-emitting diodes are finding ever wider applications. For example, in consumer applications, LED light bulbs are showing promise as replacement for conventional white light incandescent or florescent light bulbs. Further, more and more electronic devices adopt LCD as display, and LEDs are becoming increasingly popular as a backlight source.
LEDs are generally regulated by controlling their current. Typically, LEDs are operated such that their average current is constant, and power electronics components are used to create circuits which convert AC voltage into regulated LED constant average current to achieve a desired brightness. Dimming the brightness of the LED lamp may offer additional energy savings, enhance flexibility, and improve light source efficiency and lifetime. Dimming of an LED string can be controlled by either an analog dimming signal or a digital dimming signal. In analog dimming, the LED current is varied according to the magnitude of the dimming signal, but the LED is always on. In digital PWM (pulse width modulation) dimming, the LED on/off times are varied, and the LED brightness is controlled by the duty cycle of the PWM dimming signal.
It is desirable for an LED driver to be able to operate with both analog and digital dimming control signals. One conventional approach is to use two separate pins for accepting an analog dimming signal and a digital dimming signal. In some cases, two different control paths are used. A conventional approach uses a single input DIM pin to accept either an analog or a digital dimming control signal. The control circuit determines whether the input signal is analog or digital. In analog input dimming control mode, the LED current is varied according to the magnitude analog dimming control signal. In the digital input dimming control mode, the LED current is determined by the duty cycle of the input signal. For both dimming control types, the output current regulation remains analog.